Tumor angiogenesis factor is being purified from tumor cells grown in tissue culture. An attempt to generate the corresponding antibody and to characterize the active protein is in progress. Chorioallantoic membrane of chick embryo is the bioassay currently used. In vitro assays of TAF activity, using endothelial cell cultures, are being pursued. Support for the concept of tumor dormancy as a result of "anti-angiogenesis" is being developed in experiments using the chick embryo.